1. Field of the invention
This application relates to apparatus, specifically a circuit, for a highly accurate, low cost temperature sensor, particularly one using silicon thermometry and which can be implemented by an application specific integrated circuit, that also possesses a high degree of linearity and a wide dynamic range.
2. Description of the Prior Art
Accurate low-cost, linear thermometry is a recurring application that crosses a wide variety of technical disciplines. For several years, the LM-34 and LM-35 series of temperature sensors available from National Semiconductor Corporation of Santa Clara, Calif., have been predominantly used as sensing elements in temperature sensing devices designed for such applications. While these devices are relatively accurate, in some applicationsxe2x80x94particularly those intended for mass market applications, such as automobile systems, the cost of these sensors can be excessive.
Therefore, a need currently exists in the art, and has existed for some time, for a temperature sensor that is highly accurate, has a wide dynamic range, is highly linear, and which can serve as a pin-for-pin replacement for the LM-34 and LM-35 series of temperature sensors, but which provides equal or better performance than these existing sensors and, particularly advantageously, at a reduced cost.
Accordingly, my present invention advantageously overcomes the above-described deficiencies in the art through a temperature sensing circuit that employs silicon thermometry, i.e., use of a silicon diode junction to provide a highly linear temperature dependent voltage, along with subsequent signal processing stages, either mixed-mode or digital, that collectively implement, through two-point calibration, independent adjustment of slope (span or full scale) and output offsets. Zero offset adjustment is advantageously accomplished through use of a digital tear.
In both embodiments, given the inherent linearity of silicon thermometry, zero offset and desired output voltage are set, independently of each other, at a first predefined ambient calibration temperature as effectively two separate offset values, while slope (span) is set at a second predefined calibration temperature (typically a full scale temperature) different from the first temperature.
In a mixed-mode embodiment, an output voltage produced by a silicon diode junction, is additively combined, through a variable gain amplifier (VGA), to an output of a digital tear ambient temperature offset adjustment circuit that provides ambient temperature offset correction. An output of the VGA is routed both to an input of a zero crossing detector and to an input of an output amplifier. The zero crossing detector controls application of clock pulses to the offset circuit. The output amplifier additively combines the output of the VGA and an output of an ambient temperature output adjustment circuit, the latter producing in effect a second offset value, to produce an output temperature dependent voltage.
In use, the circuit is calibrated at a desired and predefined first ambient calibration temperature, the exact value not being critical, by setting the gain of the AGC to maximum and enabling the digital tear offset adjustment circuit to incrementally slew (tear) its output value until the output of the VGA is zero, at which point the zero offset value is set. Once this occurs, the ambient temperature output adjustment, i.e., the second offset, is set such that a desired voltage appears at the output of the circuit. Thereafter, at the second calibration temperature, the gain of the AGC is digitally programmed, through application of a controlled incrementing/decrementing digital value, to yield a desired output voltage at that particular temperature; hence, effecting slope (span) correction. By virtue of a highly linear temperature dependent characteristic of a silicon diode junction, essentially no linearity correction needs to be added to the circuit to achieve, across a wide operating range, operating linearity on the order of xc2x10.05 degree C and accuracy on the order of xc2x10.1 degree C.
The digital core embodiment implements the same functionality though in a digital, rather than mixed-mode domain. Here, the diode junction output voltage is converted to a digital value, then digitally processed to implement this functionality and finally converted back to an analog output voltage.
Advantageously, in accordance with a feature of my invention, the circuit enters a calibration mode, where gain and various offset values are digitally varied, and a programming mode where these values are subsequently programmed, into non-volatile memory in the circuit, in response to predefined states in the voltage levels applied to V+ (power) and output terminals; these states being detected by suitable control circuitry within the device. In particular, calibration and programming modes occur whenever a power terminal is held at one of two prescribed voltage levels, e.g., 10 and 20 volts, respectively, and ground potential is applied, for a predefined time approximately 10 xcexcS, to an output terminal of the circuit. Within the calibration mode, the gain of the VGA can be incrementally and successively increased or decreased in response to transitions then occurring in the power voltage, within a predefined time of the output signal grounding, to implement full scale (slope) correction at the second ambient calibration temperature. In particular, a positive transition from, e.g., 10 to 10.5 volts incrementally increases the gain of the VGA, while a negative transition from, e.g., 10 to 9.5 volts incrementally decreases this gain. By detecting specific temporal manipulation of the voltages applied to the power and output terminals and invoking modes and specific calibration operations as a result, the inventive circuit eliminates any need for a specialized interface and associated circuitry for accessing register to impart calibration data to the circuit;thus, reducin circuit complexity and cost. Moreover, the resulting three-pin connection,i.e., power, ground and output voltage for two,if a case housing the inventive circuit is the ground connection,advantageously enables the inventive circuit to be a direct pin-for-pin replacement for the industry standard national LM-34 and xe2x88x9235 series equivalent sensors.